The invention relates to a transmission system comprising a transmitter for transmitting signals in N carriers distributed symmetrically around a centre frequency f.sub.e, which signals have a spectrum comprised of a central part formed by N' transmitted carriers (N'&lt;N) and a right-hand side part and a left-hand side part formed each by (N-N')/2 non-transmitted carders, this transmitter being connected to a receiver which comprises at least a frequency synchronizer which determines a synchronization correction signal, and a local oscillator controlled by said correction signal.
The invention likewise relates to a receiver for such a system.
Such a system has applications to the transmission of signals which have a symmetrical spectrum, such as different digital and analog signals. An example of such a signal is a signal transmitted according to an Orthogonal Frequency-Division Multiplexing technique, transmitted, more specifically, by way of Hertzian waves, by cable, by satellite, for example for applications to digital television.
The signals transmitted according to an N-frequency-division multiplexing technique will in the following be denoted by the simplified name of OFDM (Orthogonal Frequency-Division Multiplex) signals. The OFDM technique consists of dividing an information signal to be transmitted into fractions by distributing it over a large number of low-rate elementary channels. Thus, a highly selective wideband channel is transformed into a large number of non-selective elementary channels. Because they all form a wideband channel, it will be hardly likely that fading during transmission will simultaneously affect the whole channel. This technique makes it also possible to reduce intersymbol interference.
Each elementary channel thus corresponds to one frequency, all the frequencies being symmetrically distributed around a central carrier frequency. As it is hard to accept the use of selective filters at the receiving end, an overlapping of the spectra is preferably tolerated, but conditions as to orthogonality between the frequencies are then imposed to eliminate the intersymbol interference at the sampling instants. The whole spectrum of an OFDM signal thus tends towards a rectangular spectrum.
To avoid problems with spectrum overlapping at the receiving end due to the sampling of the received signal, the side carriers situated on the left and right of the frequency spectrum are not transmitted. In addition, the filtering at the receiving end can be performed with less steep edges.
At the receiving end the received signals are to be demodulated and then decoded to restore the original information signals. For this demodulation a local oscillator is used whose frequency is to be controlled by the transmitting frequency by means of a synchronization correction signal.
Such a system is known from the article entitled "Digital Sound Broadcasting to Mobile Receivers", by B. Le Floch et at, IEEE Transactions on Consumer Electronics, Volume 35, No. 3, August 1989. This article describes an automatic frequency control (AFC). This arrangement operates in that it detects the phase difference between the received signal and the estimated symbol of the constellation. It may thus detect a maximum frequency offset of .+-.1/8T between the local oscillator of the transmitter and the local oscillator of the receiver where T is the symbol duration. It may be deduced therefrom, that, for example, for a symbol duration T=160 .mu.s and a carrier frequency f.sub.e =500 MHz, the lock-on range is 781.25 Hz which requires a high-precision (.DELTA.f/f.sub.e)=1.56 p.p.m. and very stable local oscillator (for the receiver).
The disadvantage of such an arrangement is thus the necessity to have a high-precision, thus very costly, local oscillator, because the maximum acquisition range is .+-.1/8T. Thus, with such synchronizers, the acquisition range is very narrow and the receiver cannot be automatically locked-on to when there is considerable loss of synchronization.
The main object of the invention is thus to increase the acquisition range of the synchronizer. This is to be realised with a local oscillator which is not very precise and is thus less costly, for example, for mass production applications to digital television.